Many trucks carry loads that vary greatly due to the loading and unloading of cargo. To carry the full range of loads, some trucks use one or more non-drive rear axles in conjunction with one or more drive axles. The auxiliary non-drive rear axles may be needed for a variety of reasons, including compliance with bridge laws, truck suspension and tire ratings, etc. In the unloaded condition, the non-drive axles are just “along for the ride,” since they are not needed to carry any load in that condition. For this reason, many trucks utilize non-drive axles that can be lifted off of the ground and into to a stored position (the “lifted position” or “raised position”) when not needed. These axles are generally referred to as “lift axles” or “liftable axles.” By raising the lift axle when the vehicle is lightly loaded, wear on the lift axle tires can be reduced and fuel economy can be increased due to the absence of rolling resistance from the lift axle tires.
Lift axles can be installed ahead of or behind the drive axles on a straight truck or tractor. If a lift axle is positioned ahead of the drive axle, the lift axle is referred to as a “pusher.” If the lift axle is positioned behind the drive axle, it is referred to as a “tag.” Lift axles can also be installed ahead of or behind an axle or tandem axles on an articulating trailer. There are both steerable and non-steerable lift axles. As the name implies, the wheels on non-steerable lift axles cannot turn, and as a result non-steerable lift axles need to be raised prior to turning a tight corner or the tires will scuff and impose high lateral stresses on the wheels and other chassis components. Steerable lift axles include wheels that caster so they can remain in contact with the ground during tight turning.
When a lift axle is in the lowered or active position, the load on the lift axle (commonly referred to as “sprung mass”) is typically carried by one or more “load springs.” The load springs are often air springs, which typically include flexible bellows that form an air chamber that is inflated to extend the spring and carry the load. Many lift axles also employ additional air springs (commonly referred to as “lift springs” or “lift bags”) to lift the axle when the vehicle is in the unloaded or lightly loaded condition. In existing lift axle systems, the lift springs and the load springs are not inflated at the same time. Instead, only one of the two spring sets, either the lift springs or the load springs, is fully inflated at any given time depending on the state of the lift axle. Various lift axle systems are described in U.S. Pat. No. 5,403,031 to Gottschalk et al., U.S. Pat. No. 6,311,993 to Hulstein et al., U.S. Pat. No. 6,880,839 to Keeler et al., and U.S. Pat. No. 9,352,628 to Barton et al., each of which is incorporated herein by reference in its entirety.